1. Field of the Invention
The invention relates to a variable gain differential amplifier, and variable degeneration impedance control device and method for use in the same, and more particularly, to a variable gain differential amplifier that has both linearity and gain adjustment.
2. Description of the Related Art
A variable gain amplifier (VGA) is an amplifier that can adjust a gain by receiving and amplifying a difference between signals (hereinafter referred to as “differential signal”).
Commercialized devices that use variable gain amplifiers may be personal digital assistants (PDAs), mobile communication devices, cellular phones, wireless two-way data communication devices, etc.
FIG. 1A is a schematic circuit diagram illustrating an example of a conventional amplifier.
The amplifier shown in FIG. 1A includes loads 101 and 103, amplifying parts 105 and 107, a variable degeneration impedance part 50, and constant-current sources 109 and 111.
The amplifying parts 105 and 107 receive and amplify differential signals Vin+ and Vin−. The loads 101 and 103 are connected between the amplifying parts 105 and 107 and a power supply VDD so as to level potentials of output signals of the amplifying parts. The variable degeneration impedance part 50 varies its impedance value depending on a control signal Vcont inputted thereto.
In the above-described conventional amplifier, an active element such as a metal oxide semiconductor (MOS) is used as the variable degeneration impedance part 50. As is generally known in the art, the MOS has a gate terminal G, a source terminal SOURCE, a drain terminal D and a bulk terminal B. The bulk terminal B is connected to ground, and a control signal for gain adjustment is inputted to the gate terminal G. By adjusting the value of the control signal, a resistance value seen from the source terminal and the drain terminal into the MOS transistor is controlled, and thus a gain level of the amplifier is adjusted.
The principle of adjusting the gain level of an amplifier using a transistor 50 will now be described with reference to FIG. 2 which illustrates a voltage-current curve of the MOS transistor. According to FIG. 1A, one of four curves shown in FIG. 2 can be selected by adjusting the value of a gate voltage Vcont of the transistor 50, and the resistance value can be adjusted by selecting the value of a drain voltage VDS of the transistor 50. Also, in order to ensure the linearity of the amplifier, the value of the drain voltage VDS of the transistor 50 is selected so that the transistor 50 operates in a triode region near its origin.
The conventional variable gain differential amplifier shown in FIG. 1A has an advantage in that the gain level of the transistor 50 can be adjusted by adjusting the resistance value of the transistor 50 using the voltage Vcont. However, the triode region L, in which the linearity is ensured, is too narrow since it is limited to the vicinity of the origin. Accordingly, if a large signal is applied between the drain and the source, the linearity of the amplifier cannot be ensured. Consequently, the conventional amplifier shown in FIG. 1A can adjust the gain level, but cannot ensure the linearity sufficiently.
FIG. 1B shows another example of a conventional amplifier. In this conventional amplifier, two MOS transistors 113 and 115 are used as resistance elements for degeneration. Differential input signals to be amplified are inputted to the gates of the respective transistors 113 and 115, and the resistance values thereof are determined according to the levels of the input signals. Although the resistance values of the respective transistors 113 and 115 are varied depending on the levels of the input signals, the added value of both the resistance values is kept constant. Accordingly, the conventional amplifier shown in FIG. 1B can ensure the linearity, but cannot adjust the gain level since the DC levels of the gain input differential signals are fixed.